When radiation passes through a small hole or narrow slit or is attenuated by a small pin or a narrow bar the resulting image "spreads" and is larger or wider than the original object and the contrast between the image and the background is reduced. This is a combined result of energy and diffraction and scatter, inaccuracies in the detection means, electronic noise and digitization errors. The exact functional shape of the "spread" is known as the "modulation transfer function" or MTF. Imaging systems have total "MTF's" that are the result of the combination of the MTF's of each of the specific spreading mechanisms in the image. In summary the images of holes and slits or pins, and bars are "spread" and of reduced contrast as compared to the actual objects.
The MTF usually defines a fixed addition to the linear dimensions and a concomitant reduction from the contrast of any object. Thus, when the slit or bar or hole or pin is large enough the average size and contrast are for practical purposes unchanged from that of the actual object. When the objects are small, the average contrast in the image is proportional to both the objects contrast and width.
There have been many attempts to correct for the MTF spreading. For example, complicated "restoring" filters which attempt to resharpen the spread images have been used. The presently used restoring filters are time-comsuming and "noisy". Hence they enhance the noise already present in the image and thus tend to detract from the system efficiency and the image quality.
In many studies using image enhancing techniques the slight enlargement of the object is not as much of a problem as is the deterioration in contrast caused by the MTF generated spreading.
For example, in digital fluorography and similar studies where the vascular system is being studied; it is of little consequence to the detectability of the blood vessels when the image of the blood vessels are slightly enlarged. Whereas, if the contrast is reduced it becomes impossible to detect or distinguish the smaller blood vessels.
One solution to the MTF generated spreading problem is to use a different "window" (grey scale stretching) for viewing the smaller blood vessels. The "solution" results in a more contrasty scale, which loses vital details in larger vessels. Also, when using this "solution" the "correct" window has to be found manually; a time consuming process. Another attempted solution to the MTF generated spreading problem is to use a nonlinear scale. For example the (double window), comprising two separate linear parts covering the large vessels and the small vessels, respectively. Other nonlinear scales (eg. exponential and logarithmic scales) have been used but with no theoretical background they tend to distort the relative contrast, lose details and make densitometric calculations impossible.
Thus there is a real long-felt need to correct for the contrast deterioration of images caused by MTF generated spread of the objects in the images.